In outline, such a module comprises a vessel or casing receiving said rigid elements which may be membranes, or membrane supports. These elements which are made of a rigid substance such as a ceramic, glass, carbon, or a metal, are referred to below as "active" elements. More precisely, a module comprises:
a vessel which is generally cylindrical in shape and made of a metal or a plastic material optionally reinforced by fibers; and
one or more active elements which are generally tubular or quasi-tubular in geometry, in particular of the multi-channel type, such as described in the following publications:
J. Gillot and D. Garcera--"Nouveaux Media Filtrants Ceramiques pour Microfiltration Tangentielle et Ultrafiltration" (New Ceramic Filter Media for Ultrafiltration and Tangential Microfiltration) Conference FILTRA 84--Ste Francaise de filtration October, 1984 Paris; and PA1 J. Gillot--G. Brinkman--D. Garcera--"New Ceramic Filter Media for Crossflow Microfiltration and Ultrafiltration", Fourth World Filtration Congress 22-24 April, 1986--Ostende, Belgium; PA1 mechanically support the two ends of each active element and maintain them in position; PA1 separate the fluid treatment installation into an upstream region and a downstream region relative to the membrane; and PA1 provide sealing between these two regions; and
and which are generally much longer along the channel than across the channel. These active elements are assembled with the axis of the, or each, channel running parallel to the axis of the vessel. The membrane covers the inside surface of the channel or channels in these elements, or in some circumstances the outside surface when the elements are tubes;
a set of two end plates (which are generally very rigid) together with seals which:
parts situated inside or at the periphery of the bundle of active elements and extending parallel thereto, with said parts serving to hold the end plates together mechanically. These parts do not have any active role to play with respect to fluid treatment per se, and are referred to below as props. They are required only when the module has a large number of active elements.
Such a module must be capable of operating for several years without being disassembled. Unfortunately, when the module is subjected to major temperature variations, in particular when it is assembled at ambient temperature but used at a high temperature or at a low temperature, differences in longitudinal expansion appear between the active elements on the one hand and the vessel and its props on the other. These differences in expansion may be as much as one or more millimeters when the active elements are about one meter long, which is commonly the case. In general, this difference in expansion cannot be avoided since the materials from which the active elements are made and from which the vessel and its props are made are normally different since they satisfy different requirements. When active elements made of porous ceramic material are mounted in a metal vessel having a coefficient of expansion which is generally greater than the ceramic, high temperature use tends to put the ceramic elements under tension, which they withstand poorly.
In prior assemblies, the expansion difference is taken up in two different ways.
When the end plates are fixed to the vessel, and also when props are present, the difference in expansion must be absorbed by the seals situated between the ends of the active elements and the end plates. This means that each element should have, at least at one end:
either a sliding seal, in which case sealing is difficult to maintain if thermal cycling also takes place, particularly when the fluid to be treated contains particles in suspension (i.e. filtration) since the particles penetrate between the facing surfaces of the seal;
or else flexible seals are provided, e.g. of the elastomer type or of the metal bellows type, in which case they are very expensive. If the seal is of the elastomer type, then it must be sufficiently wide laterally for the angular deformation due to a longitudinal expansion of several millimeters to remain acceptable, and this means that a bundle of active elements takes up too much room;. In addition, there are then problems associated with the nature of elastomers: no elasticity at cryogenic temperatures; and poor mechanical and chemical performance at high temperatures over long periods of time in the presence of the fluids to be treated.
When there are no props and the module contains a plurality of active elements, the differential expansion can be absorbed by a flexible or a sliding seal situated at the periphery of one of the end plates, between said plate and the vessel. However, this solution suffers from the drawback of applying forces on the seals between the active elements and the end plates due to the difference in fluid pressure on opposite sides of the end plates; in other words the seals are subjected to shear. In addition, these forces are then transmitted to the active elements themselves which are subjected to traction or to compression. When tangential filtration is being performed with frequent backwashing of the membranes by pressure reversal, then the filter elements are cyclically subjected to traction and to compression, and this is most unfavorable.
The object of the invention is thus to provide a module structure in which the end plates and the seals are required to absorb small residual expansion differences only, e.g. due to non-uniform temperature distribution within the module.